The present invention relates generally to techniques for reducing the effects of jamming in radio-frequency receivers and more specifically to methods and apparatus for enhancing the reception of global positioning system (GPS) signals in the presence of jamming signals.
Global position systems, such as the American NAVSTAR GPS and Russian GLONASS, are known. The NAVSTAR GPS developed by the U.S. Department of Defense is a satellite-based radio navigation system which transmits information from which extremely accurate navigational calculations can be made in three-dimensional space anywhere on or near the Earth. Three-dimensional velocity can be determined with similar precision. The GPS uses eighteen to twenty-four satellites that may, for example, be evenly dispersed in three, inclined, twelve hour circular orbits chosen to ensure continuous twenty-four hour coverage world-wide. Each satellite uses extremely accurate cesium and rubidium vapor atomic clocks for generating a time base. Each satellite is provided with clock correction and orbit information by Earth-based monitoring stations.
Each satellite transmits a pair of L-band signals. The pair of signals includes an L1 signal at a frequency of 1575.42 MHz and an L2 signal at a frequency of 1227.6 MHz. The L1 and L2 signals are bi-phase signals modulated by pseudo-random noise (PRN) codes and an information signal (i.e., navigation data) encoded at 50 Hz. The PRN codes facilitate multiple access through the use of a different PRN code by each satellite.
Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. The ephemeris data is used in conjunction with a set of Keplerian equations to precisely determine the location of each satellite. The receiver measures a phase difference (i.e., time of arrival) of signals from at least four satellites. The time differences are then used to solve a matrix of four equations. The result is a precise determination of the location of the receiver in three-dimensional space. Velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured frequencies are used to determine Doppler frequency shifts caused by differences in velocity. The measured differences are used to solve another set of equations to determine the velocity based upon the Doppler phase shift of the received signal.
GPS signals are very low in amplitude and are transmitted using a spread-spectrum signal bandwidth centered at about 1575.42 MHz. The GPS signals cover a frequency spread of about 20 MHz. GPS receivers are subject to disruption by jammer signals, which may be transmitted either as narrow band signals or broadband signals. Known GPS receiver systems may reduce the affects of narrow band jamming by using frequency-selective filters, such as notch filters, to attenuate the jamming signal. However, broad band jamming signals are more difficult to reduce or eliminate (to "null-out") as the frequency spread of the jamming signals approximates the frequency spread of the GPS signal. However, because the frequency spreading sequence of the GPS signal is encrypted according to a pseudo-random noise code, the jammer cannot be precisely synchronized to the GPS signal. This permits the effects of the jamming signal to be reduced by nulling-out the jamming signal. Further, the signal strength of the jamming signal is typically much greater than the signal strength of the GPS signal and tends to "stick-up" above the GPS signal.
Some known jamming nulling techniques are based on determining an angle of arrival of a signal based on the phase shift of the signal observed at different antenna elements of an antenna array. Various "weights" or "weight values" are assigned to each antenna element and are used to adjust the phase and level of attenuation of the received signal in an attempt to null-out the jammer signal. Power minimization is a known technique that attempts to adjust the weights so as to reduce the total measured power coming from the antenna array. Power minimization techniques rely on the assumption that the jammer signal is much stronger than the GPS signal and that it emanates from a different direction. An array having multiple antenna elements is spatially selective so that a null can be placed in the direction of the jammer by adjusting the weight values. Various known algorithms, such as least mean squares and direct matrix inversion may be used to implement the power minimization technique in digital systems. It is very difficult to use such algorithms in analog systems. In power minimization techniques, it is assumed that almost all of the signal power is due to the jammer component, and not due to the GPS signal, because the GPS signal is very weak and therefore provides no significant power contribution.
In power minimization techniques, weights are used to constrain the signals. However, if the weights associated with each antenna element are allowed to be adjusted to minimize the power level of the received signal, all the weights would eventually be driven to a zero level in an attempt to minimize the overall power, and no signal could thus be obtained. Accordingly, in one example of a known power minimization technique, one of the weights is constrained so that it is equal to one (or some predetermined fixed value) and cannot be adjusted. Therefore, the above-described power minimization technique dictates that less than all of the weights may be adjusted.
In accordance with known power minimization techniques, in one situation, the weights are adjusted according to a predetermined pattern, for example, beginning at some starting point, the gain and the phase are increased or decreased by a predetermined step size. Although not necessarily a random pattern, if the results are beneficial, adjustment to the weights continues in the same direction. This eventually approaches a solution that provides the receiver, such as the GPS receiver, with a somewhat improved signal. However, the improved signal is not optimal. For power minimization techniques, the measure of the improved signal is represented by a signal having the lowest overall power level.
A significant problem associated with power minimization techniques is that it does not make use of information relating to the position of the satellite relative to the receiver or antenna of the receiver. Therefore, nulls may be inadvertently placed in the direction of the satellite, and the desired GPS signal may be adversely degraded. Additionally, power minimization techniques make no attempt to boost gain of the received GPS signal in the direction of the satellites.
Techniques exist to steer gain to desired signal sources. Typically, a set of weights may be selected to steer gain toward a single desired signal source. Because the nature of GPS involves multiple simultaneous desired signal sources, straightforward application of existing techniques implies multiple sets of weights for multiple signal sources. A set of weights means that one weight exists for each antenna element. In other known systems, such as digital GPS systems, each antenna output is sampled and digitized prior to processing. To implement multiple sets of weights, each antenna element is associated with a separate weight, where the set of weights for the antenna array corresponds to one GPS satellite. However, use of multiple weights is very expensive and such systems incur very large hardware burdens. Use of multiple sets of weights would require an additional hardware path for each group of weights corresponding to a specific satellite in such digital systems. Each path would require high-speed hardware, such as multipliers, delay circuits, and filters in addition to high speed processing devices. The same path cannot be used for multiple satellites because processing is not a sequential process and is limited by the processing speed of the hardware associated with each path. Thus, if the same hardware were to be used with multiple sets of weights, for example, four sets of weights, the hardware would need to process signals at four times the rate of hardware dedicated to a single set of weights. Thus, for each set of weights utilized, an additional hardware path with dedicated hardware must be provided. As such, digital systems using multiple sets of weights are hardware intensive and expensive to implement given the current state of technology.
With respect to analog systems, use of multiple sets of weights is prohibitive. To implement multiple hardware paths dictated by the use of multiple sets of weights, the received signal must be "split" and provided to each of multiple processing paths. The cost and complexity of such an analog system renders implementation of multiple sets of weights undesirable. Accordingly, known GPS analog systems are restricted to use of a single set of weights due to size, cost, and complexity considerations. Such systems may inadvertently place spatial nulls in the direction of the satellite, which may degrade system performance.
Accordingly, it is an object of the present invention to provide a novel beam-forming method for reducing jamming to substantially overcome the above-described problems.
It is another object of the present invention to provide a novel technique for reducing jamming that reduces or eliminates placement of spurious nulls on the desired signal.
It is a further object of the present invention to provide a novel technique for reducing jamming in an analog GPS.
It is also an object of the present invention to provide a novel technique for reducing jamming in an analog GPS while simultaneously boosting the gain of the received GPS signal.
It is still an object of the present invention to provide a novel beaming-forming technique in an analog GPS system utilizing a single set of complex weights optimized for multiple satellites.
The disadvantages of present jamming reduction and power minimization techniques are substantially overcome with the present invention by providing a method for reducing jamming by beam forming using spatial nulling.
The present invention is advantageous in analog GPS systems and is based on optimizing the average signal-to-noise ratio (SNR) averaged over the number of satellites tracked. The gain of the GPS signal received is boosted in addition to nulling-out the jammer signal. This is referred to as a "beam-forming" technique. Beam-forming techniques have not been successfully used in analog systems because in order to boost the gain of the received signal, the position of the satellite must be known, and such an implementation would require multiple sets of weights.
In one embodiment of the present invention, beam-forming is accomplished in an analog system using only a single set of weights for the antenna array optimized for the GPS satellites tracked. To accomplish this, positional information with respect to the satellites may be used, for example, in the form of existing navigational data. Many analog GPS systems are based on platforms that include navigational systems (NAV systems), such as inertial navigation systems (INS). The NAV systems provide information with respect to the orientation of the platform or orientation of the antenna relative to the earth. Additionally, information as to the position of the GPS satellite is available via almanac tables, which may be periodically updated.
Using the NAV information, the orientation of the received signal of the satellite may be determined. In many systems, at some point, the location of the GPS receiving system is known, for example, as determined by the NAV system. Thus, for example, if the GPS system becomes temporarily disrupted due to jamming, the NAV system can provide the required positional information.
Accordingly, in one embodiment of the present invention, a method of reducing jamming in a global positioning system (GPS) satellite receiving system includes the steps of: a) selecting an initial weight value corresponding to each antenna element of an antenna array; b) selecting a weight adjustment scheme for adjusting the weights; c) measuring a power output from the antenna array; d) obtaining navigational data representing the orientation of the array; e) calculating a gain of the antenna array corresponding to each of the GPS satellites, using the navigational data to provide an indication of the orientation of the array relative to each GPS satellite; f) estimating or measuring a power level of a received signal corresponding to each GPS satellite; g) solving for a signal to noise ratio for each GPS satellite, using the estimated power level corresponding to each GPS satellite; h) iteratively and continuously adjusting the weights to obtain a greatest value of the signal to noise ratio; and i) continuously repeating steps (c) through (h).